The present invention relates to a motor control apparatus for controlling of an applied voltage to a motor by sensing phase current state of the motor without using a current sensor, and to a motor unit including such a motor control apparatus.
Conventional motor control apparatus will be described below using two examples.
(1) As a first type of conventional motor control apparatus, there is known a three-phase bridge inverter of a plurality of switching elements and current-circulating diodes connected in parallel with the switching elements, respectively. The motor connected to such an inverter is not provided with a position sensor such as an encoder. In the conventional control apparatus thus constructed, both the upper and lower switching elements are placed in a cut-off state (OFF) to create a non-energization state, and a period of 60 electrical degrees, during which current becomes zero, is provided. By virtue of the provision of such a period in which current becomes zero, a condition is created so that an induced voltage, which is induced by the magnetic poles of a rotor, becomes detectable. The induced voltage thus detected is compared with a specific set value and, based on the compare result, a position of the rotor is detected. According to the rotor position thus detected, this conventional control apparatus carries out to switch the applied voltage of the phases. This prior art is of an energization method that is called the six-step drive or 120 degree conduction, and a current flowing through a stator coil has a current waveform having a rectangular shape. Techniques of the type describe above have been disclosed, for example, in Official Gazette of Japanese Published Unexamined Patent Publication No. Hei 02-32790, Official Gazette of Japanese Published Unexamined Patent Publication No. Sho 61-112590, and Official Gazette of Japanese Allowed Patent Publication No. Sho 59-25038.
(2) As a second prior art for motor control apparatus, techniques have been reported, for example, in reports of the Japanese Institute of the Electric Engineering (T. IEE Japan, Vol. 115-D, No. 4, 1995; pp. 420 (April 1995); and T. IEE Japan, Vol. 110-D, No. 11, 1990; pp. 1193 (November 1990)). In the second prior art, neither an induced voltage is detected with a current zero period created, nor a position sensor is used for direct detection of a rotor position. In the second type conventional technique, however, a current sensor is provided which detects a current flowing in a corresponding coil, and a current value detected by the current sensor and a command voltage value are used to estimate a rotor position .theta. from a motor theoretical equation as the need arises. The estimated position .theta. serves to form a 180 degree energization command waveform, and continuous 180 degree (sine wave) energization drive is carried out.
Next, the first prior art described in the foregoing (1) part will be described in detail.
FIG. 20 is a block diagram showing the structure of a commonly used motor control apparatus.
Referring to FIG. 20, the motor 100 has stators (not shown in the figure) and a rotor 200. A coil 300 is wound around each stator through which phase current flows. The stator coil 300 is made up of a u-phase coil 300u, a v-phase coil 300v, and a w-phase coil 300w. Secured on a surface of the rotor 200 is a permanent magnet. Each coil 300 is coupled to an inverter 400 for the controlling of phase voltage application to each coil 300.
As shown in FIG. 20, the inverter 400 has a direct current power source 50, an upper switching element group 60 (61u, 61v, 61w), and a lower switching element group 70 71u, 71v, 71w. The upper and lower switching element groups are respectively groups of upper side and lower side in the figure and are of three-phase-bridge configuration. In addition, the inverter 400 further has diodes 81u, 81v, 81w, 82u, 82v, and 82w connected in parallel with the switching elements 61u, 61v, 61w, 71u, 71v, and 71w in the upper switching element group 60 and the lower switching element group 70, respectively. The upper switching element group 60 and the lower switching element group 70 are coupled to a switching element modulation circuit 109, and they are controlled by the switching element modulation circuit 109. Additionally, the conventional control apparatus includes a voltage output circuit 110 formed of resistive elements, an induced voltage detection circuit 113d for the detection of an induced voltage that is induced by the coil 300, a voltage command part 120, a second cut-off period command part 150, and a fourth applied voltage control circuit 152.
The inverter 400, by which the motor 100 is drive-controlled, is provided with the direct current power source 50 whose positive and negative sides are at E[V] and at 0 V, respectively. The upper switching element group 60 forms current paths from the direct current power source 50 to the coil 300 of three phases, namely a u-phase, a v-phase, and w-phase. On the other hand, the lower switching element group 70 forms current return paths from the three-phase coil 300 to the direct current power source 50. The diodes 81u, 81v, 81w, 82u, 82v, and 82w are connected in reverse parallel with their corresponding switching elements, respectively.
Next, the operation of the conventional control apparatus constructed as described above will be illustrated in detail.
The switching element modulation circuit 109 sends to the upper switching element group 60 as well as to the lower switching element group 70 a conduction (ON) command signal or a cut-off (OFF) command signal for control of the energization voltage to the coil 300 of the coils 300u, 300v, and 300w.
FIG. 21 is a waveform diagram depicting timings of the switching elements and applied voltages in the conventional control apparatus. FIG. 21, comprised of parts (a)-(f), shows conduction (ON) and cut-off (OFF) signals with respect to the upper switching element group 60 (61u, 61v, 61w) and the lower switching element group 70 71u, 71v, 71w. In the parts (a) to (f) of FIG. 21, "HIGH LEVEL" represents the ON state and "LOW LEVEL" represents the OFF state. Accordingly, in a period T1, the upper switching element 61u repeatedly switches on and off by pulse width modulation (PWM), while the lower switching element 71v is placed in the ON state. At this time, the remaining other switching elements 61v, 61w, 71u, 71w are all placed in the OFF state. As a result, the upper switching element 61u of the u-phase and the lower switching element 71v of the v-phase conduct, thereby causing a current to flow from the stator coil 300u of the u-phase to the stator coil 300v of the v-phase.
Likewise, in a period T2, the upper switching element 61u repeatedly switches on and off by PWM, while the lower switching element 71w is placed in the ON state. At this time, the remaining other switching elements 61v, 61w, 71u, and 71v are all placed in the OFF state. As a result, there is a current flow from the u-phase coil 300u to the w-phase coil 300w.
Likewise, in a period T3, there is a current flow from the coil 300v to the coil 300w. In a period T4, there is a current flow from the coil 300v to the coil 300u. In a period T5, there is a current flow from the coil 300w to the coil 300u. In a period T6, there is a current flow from the coil 300w to the coil 300v.
As described above, the timing of the conduction (ON) and cut-off (OFF) in the periods T1-6 is repeated to control phases that are electrically energized for every 60 electrical degrees, causing the rotor 200 to rotate. In this case, currents, which flow through the coils 300u, 300v, and 300w, have individual waveforms that differ in phase from one another by 120 electrical degrees. FIG. 22 is a waveform diagram depicting a phase induced voltage induced in a phase at that time and a waveform of a phase current flowing in the phase.
The period from the start of the period T1 up to the end of the period T3 represents a 180 electrical degree period. The control apparatus, shown in FIG. 20, is fed a command so that, in each of the phases, voltage is applied in a period of 120 degrees of the 180 electrical degree period. Accordingly, this control apparatus is called the 120 degree energization. Alternatively, the control apparatus is called the six-step drive because phases which are electrically energized are switched for every 60 degrees of the 360 electrical degrees.
These conduction and cut-off control periods have been given beforehand by the second cut-off period command part 150 (FIG. 20).
Next, a way of obtaining a timing signal for the switching between the periods T1-6 in the conventional control apparatus will be described.
In the first place, the voltage output circuit 110 detects voltages (Vu, Vv, Vw) applied to input/output terminals of currents to the coil 300 located between the upper and the lower switching elements 61u, 61v, and 61w and 71u, 71v, and 71w of the respective phases.
Parts (g)-(i) of FIG. 21 are waveform diagrams showing the terminal voltages Vu, Vv , and Vw of the respective phases of the coil 300.
Each of the terminal voltages in the period T1 will be discussed. The u-phase terminal voltage Vu is almost the voltage E[V] of the direct current power source 5 when the upper switching element 61u turns on. On the other hand, when the upper switching element 61u turns off, the terminal voltage Vu is 0 V because a current flows through the diode 82u. In the period T1, the v-phase terminal voltage Vv is almost 0 V because the lower switching element 71v is in the ON state.
At the beginning of the period T1, a current flows in the coil 300w through the diode 82w. During this period, the w-phase terminal voltage Vw is 0 V. Then, after the current becomes zero, an induced voltage appears in the w-phase when the upper switching element 61u is in the ON state. At this instant, there is created a state capable of detecting induced voltages. It is to be noted that, when the upper switching element 61u is in the OFF state, it is impossible to detect an induced voltage. Accordingly, by providing a cut-off control period of 60 electrical degrees for cutting off energization and causing the current to become zero, it becomes possible to detect an induced voltage resulting from the rotation of the rotor 200. The terminal voltage Vw when the upper switching element 61u is in the ON state will vary with the rotation of the rotor 200. In other words, terminal voltage detection makes it possible to detect a rotation position of the rotor 200. It is to be noted that, when the w-phase induced voltage in the T1 interval is ew, a voltage, expressed by 3ew/2+E/2, is output as a terminal voltage. A relationship between the induced voltage and the rotation position of the rotor 200 is stated in detail in the previously-mentioned Official Gazette of Japanese Published Unexamined Patent Publication No. Hei 02-32790.
Based on the induced voltage thus detected, the timing of energization for each coil 300 is controlled as follows.
A digital method of instantly detecting an induced voltage will be described concretely.
In the period T1, there is made a comparison between an output induced voltage of the w-phase (ew) and a preset reference voltage (E/2). When the induced voltage (ew) crosses the reference voltage (E/2), this causes the induced voltage detection circuit 113d to provide a zero cross signal. When a voltage lead angle .alpha. from the output timing of a zero cross signal is 0 degree, a timing, which is advanced in electrical angle by 30 degrees with respect to the zero cross signal output timing, is made to serve as a subsequent commutation timing and is determined to be the start point of the period T2.
Actually, the terminal voltages in each coil 300 are voltage-divided by resistive elements. A voltage-divided induced voltage is compared with a reference voltage corresponding to that voltage-divided induced voltage. An output zero cross signal as a result of comparison with the induced voltage is fed into a computer and is processed. It is to be noted that a timing which is advanced 30 degrees in electrical angle can be calculated easily using a timer within a computer.
As described above, by comparing an induced voltage with the reference voltage, a zero cross signal will be output at an early stage when the rotation speed of the rotor 200 becomes fast. As a result, a change in the phase to be electrically energized is made early according to the output zero cross signal. By detecting an induced voltage in the way described above, application of a voltage is carried out at a timing according to the rotor position of the motor 100.
The above-described technique may be defined as follows. When the fourth applied voltage control circuit 152 provides a cut-off control signal for w-phase switching elements, the induced voltage detection circuit 113d detects the terminal voltage Vw of the w-phase provided from the voltage output circuit 110 in a link operation with a switching signal of a different phase provided from the switching element modulation circuit 109. The induced voltage detection circuit 113d outputs a zero cross signal in the event that the detected induced voltage intersects the reference voltage.
Next, according to the zero cross signal from the induced voltage detection circuit 113d and a cut-off command period of 60 degrees from the second cut-off period command part 150, the fourth applied voltage control circuit 152 provides, to the switching element modulation circuit 109, a conduction/cut-off control signal so that the switching enabling and disabling of given switching elements is controlled. For example, when the voltage lead angle .alpha. is 0 degree, the fourth applied voltage control circuit 152 provides a switching cut-off control signal of the lower switching element 71v and a switching conduction control signal of the lower switching element 71w at a timing which is advanced 30 degrees in electrical angle to the switching element modulation circuit 109.
As described above, the fourth applied voltage control circuit 152 sequentially produces commutation timings for the periods T1-6 while the motor 100 is rotating. Also in the periods from T2 to T6, phases to be electrically energized change as in the above; however, it is possible to detect the timing of commutation by the same control method as the foregoing control method.
The fourth applied voltage control circuit 152 provides to the switching element modulation circuit 109 a conduction/cut-off control signal at the individual timings T1-6 to each phase. Then, the switching element modulation circuit 109 provides a conduction/cut-off signal in a PWM cycle only when a conduction control signal is input, and performs actual conduction/cut-off operations on switching elements. Because of this, the fourth applied voltage control circuit 152 exerts switching element control at a higher level in comparison with the switching element modulation circuit 109.
In other words, the fourth applied voltage control circuit 152 continuously provides a conduction control signal in the periods T1 and T2 of the part (a) of FIG. 21, which however does not mean that the fourth applied voltage control circuit 152 places the switching element 61u in the conducting state all the time. In the following description, the fourth applied voltage control circuit 152 sends a conduction/cut-off control signal to the switching element modulation circuit 109 and the switching element modulation circuit 109 sends a conduction/cut-off control signal to each of the switching elements of the inverter 400 in a PWM cycle.
Further, from the interval between zero cross signals sequentially provided from the induced voltage detection circuit 113d, a rotation speed of the rotor is detected. The voltage command part 120 produces a phase voltage command Vh by proportional-plus-integral operation of the difference between the detected rotation speed and a target speed and provides it (not shown in the figure). Further, the voltage command part 120 provides, based on the detected speed, the voltage lead angle .alpha..
Subsequently, the phase voltage command Vh and the voltage lead angle .alpha. both provided from the voltage command part 120 are fed to the fourth applied voltage control circuit 152. The fourth applied voltage control circuit 152 provides, to the switching element modulation circuit 109, a conduction/cut-off control signal of each of the switching elements for every 60 electrical degrees, a conduction/cut-off change timing control signal for every 60 electrical degrees, and a phase voltage command Vsou when performing PWM switching operations in a 60 electrical degree period. Then, the switching element modulation circuit 109 actually performs a pulse width modulation operation according to the phase voltage command Vsou and a conduction/cut-off operation on each of the switching elements for every 60 electrical degrees, for the application of voltage to each coil 300 of the motor 100.
Here, when the phase voltage command Vsou is great in the switching element modulation circuit 109, the width of an on-duty Ton of the switching element 6u in the period T1 shown in the part (a) of FIG. 21 will increase resulting in application of a high voltage thereto.
As described above, the preparation of phases into which no current flows causes an induced voltage corresponding to the rotation position of the rotor 200 to appear in a terminal voltage of the motor 100. Then, the zero cross position of the induced voltage and the reference voltage is detected to perform a phase commutation, whereby it becomes possible to control the rotation of the motor in synchronization with the rotation position of the rotor 200. Such a period of 60 degrees is required to bring the current back to zero and to detect an induced voltage zero cross position even when the rotor 200 greatly varies in position.
In accordance with the above-described conventional structure, however, the detecting of an induced voltage at each coil 300 requires forced formation of a 60 degree cut-off period such as the periods T3 and T6 (during which the switching elements 61u and 71u are not electrically energized) in the u-phase, the periods T2 and T5 in the v-phase which are non-energization periods, and the periods T1 and T4 in the w-phase which are non-energization periods, as shown in FIG. 21. Consequently, in the conventional control apparatus, 120 degree energization is inevitable, as a result of which neither 180 degree continuous energization nor wide-angle (in excess of 120 degrees) energization becomes impossible to carry out.
FIG. 22 is a waveform diagram depicting a phase induced voltage and a phase current in 120 degree energization in a conventional control apparatus. In conventional control apparatus, a phase current exhibits a waveform as shown in FIG. 22, therefore resulting in an increased torque ripple. This produces the problem that the degree of vibration of the motor 100 increases, and efficiency drops.
As a system intended to provide a solution to the above-described problem, there has been proposed the foregoing conventional technique described in the column (2) of the prior art description part. In accordance with this prior art, current sensors are separately provided which directly detect currents flowing in respective coils, and by making use of a current value detected by such a current sensor and a command voltage value, the rotor position .theta. is estimated from a motor theoretical equation as the need arises. This prior art control apparatus is a method of producing a series of current commands from an estimated rotor position .theta. to perform 180 degree (sine wave) energization drive. This method employs no non-energization periods, thereby solving the foregoing problem, but it requires the provision of current sensors, therefore resulting in producing another problem that the costs will increase.
Accordingly, in the conventional control apparatus, it is impossible to carry out continuous energization or an energization angle near to 180 degrees at high efficiency without the provision of any current sensors.